(1) Field of the Invention
This invention relates to improved sulfonated lignin surfactants and their use as dispersants in dyestuffs. More particularly, this invention relates to lignin surfactants having reduced staining characteristics produced by a two-step process involving blocking the phenolic functions followed by oxidative reactions.
(2) Description of the Prior Art
Sulfonated lignins and lignosulfonates from kraft and sulfite pulping processes have long been recognized for their excellent dye dispersant properties, but their dark color has excluded them from use with many fabrics since this may produce slight staining or darkening of sensitive shades on textiles. When sulfonated lignin or lignosulfonate is used as a dye dispersant, it is ball-milled with a dye cake and the mixture is then used for dyeing natural or synthetic fibers. During the dyeing process, some of the lignin can absorb onto the fabric fibers distorting the true color of the dye. The magnitude of the problem depends on the color of the lignin material.
Lignin in its natural state is almost colorless. The cause of the brown color of industrial lignins and lignosulfonates and the mechanism for the formation of chromophores during the pulping process are not completely known, although numerous suggestions have been made over the years. Based on the results of three approaches, in Advanced Chemical Series, Vol. 59, 75 (1966), Falkehag et al. proposed the following chromophores to be the potential color causing structures in kraft lignin: (1) CH.dbd.CH double bonds conjugated with the aromatic ring (as in stilbenes), (2) quinone methides and quinones, (3) chalcone structures, (4) free radicals, and (5) metal complexes with catechol structures. In Tappi, Vol. 54 (No. 10) 1680-4 (1971), Imsgard et al. estimated that although spruce milled wood lignin contains only about 0.7% o-quinoid structures, these structures can account for as much as 35-60% of the ultraviolet light absorption of lignin at 457 nm. In any event, INDULIN AT, a kraft wood lignin, has an absorptivity at 457 nm of 0.91 (g.sup.-1)(1)(cm.sup.-1). This is more than 15 times the value for spruce milled wood lignin, 0.06 (g.sup.-1)(1)(cm.sup.-1).
The occurrence of quinoid structures in the kraft pulping process is widely acknowledged to be caused by a nucleophilic attack on guaicol (softwood) or syringyl (hardwood) structures by OH.sup.- and SH.sup.- ion species. Demethylation reactions resulting in catecholic compounds easily undergo auto-oxidation to yield colored o-quinoid structures. The latter may give a variety of secondary reactions such as cycloaddition and polymerization yielding further chromophoric systems, as taught by Musso and Dopp in Chemische Berichte, 100:3627 (1967). ##STR1##
It has been shown that the color of lignin may be reduced to some degree by blocking the free-phenolic hydroxyls in lignin. Several blocking methods have been set forth, such as in U.S. Pat. No. 3,672,817 where the lignin color was reduced as much as 44% by blocking the phenolic hydroxyl with an alkylene oxide or a halogen-containing alkyl alcohol. In U.S. Pat. No. 3,763,139, lignin color was reduced by blocking the phenolic hydroxyl with reagents, such as chloromethane sulfonate, chloromethane phosphonate, 2-chloroethanol and the like. In U.S. Pat. No. 3,769,272, lignin color was reduced by blocking with 3-chloro-2-hydroxypropane-1-sulfonate. And in U.S. Pat. No. 3,865,803, the phenolic hydroxyl was blocked with an agent of the type X(CH.sub.2).sub.n Y, where X is a halogen, activated double bond, epoxide ring, or a halohydrin, Y is a sulfonate, phosphonate, hydroxyl, sulfide, or a secondary or tertiary amine, and (n) is an integer from 1 to 5.
Although each of the above methods gave some reduction of the color of an alkali lignin or lignosulfonate, none reduced the color to a sufficiently desirable extent.